1. Field of the Invention
The present invention relates to an image display medium using a particulate material which allows repeated rewriting and an image forming device.
2. Description of the Related Art
As image display media enabling rewriting there have heretofore been proposed display techniques such as twisting ball display (two-color particle rotary display), electrophoresis, magnetophoresis, thermal rewritable medium and liquid crystal having memory properties. These display techniques are excellent in image memory properties but are disadvantageous in that they cannot use a white display such as paper and thus provide a low contrast.
As a display technique using a toner which solves these problems, there has been proposed a display technique involving the enclosure of an electrically-conductive colored toner and a white particulate material in the gap between opposing electrode substrates. In accordance with this display technique, electric charge is injected into the electrically-conductive colored toner via a charge-transporting layer provided on the inner surface of the electrode on the non-display substrate. The electrically-conductive colored toner into which electric charge has been injected moves toward the display substrate disposed opposed to the non-display substrate under the application of an electric field across the electrode substrates. The electrically-conductive colored toner is then attached to the inner side of the display substrate to make contrast from the white particulate material, causing image display (Japan Hardcopy""99 Bulletin, pp. 249-252). In this display technique, the image display medium is entirely composed of a solid material. Thus, this display technique is excellent in that the display of white and black (color) can be theoretically switched by 100%. However, this display technique is disadvantageous in that there is an electrically-conductive colored toner which doesn""t come in contact with the charge-transporting layer provided on the inner surface of the electrode on the non-display substrate or an electrically-conductive colored toner isolated from other electrically-conductive colored toner. Since no electric charge is injected into these electrically-conductive colored toners, they cannot move even under the action of an electric field and thus remain at random on the substrates, giving a low contrast.
In order to solve these problems, Japanese Patent Application No. 2000-165138 proposes an image display medium comprising a pair of substrates and a plurality of kinds of particles having different colors and chargeabilities enclosed in the gap between the substrates such that they can move between the substrates under the application of an electric field applied across the substrates. In accordance with this proposal, a high whiteness degree and contrast can be attained. The applied voltage required for the display of black-and-white image is several hundreds volt. In the constitution of the particulate materials thus proposed, the required applied voltage is lowered, making it possible to expand the degree of freedom of design of the driving circuit. However, under the recent circumstances requiring further improvements of performance, further improvements of performance have been demanded. Thus, it has been desired to lower the required driving voltage for the purpose of further enhancing the stability and uniformity of image density, the stability of density contrast and the degree of freedom of design of driving circuit.
The invention is intended to solve the problems of the related art and attain the following aim. In other words, an object of the invention is to provide an image display medium which can use a low predetermined driving voltage and shows a small change of image density and image uniformity and a stable density contrast even after prolonged repetition of rewriting and an image forming device therefor.
The inventors made extensive studies. The inventors paid attention to instabilization of charged amount due to the increase of adhesion between particles and between particles and substrate or triboelectrification of particles and deterioration of efficiency in separation and movement of particles due to fluidity of group of particles charged by mutual friction. As a result, it was found that the foregoing problems can be solved by eliminating these factors. The invention has thus been worked out. According to the invention there is provided an image display medium having a pair of substrates disposed opposed to each other, and a particle group having at least two kinds of particles enclosed in a gap between the pair of substrates, in which at least one of the at least two kinds of particles can be positively charged, at least another one of the at least two kinds of particles can be negatively charged, the one and the another one have different colors from each other, and both the one and the another one has shape factors satisfying 100 less than the shape factorsxe2x89xa6140, where the shape factor=(L2/S)/4xcfx80xc3x97100; S is area of the particle; and L is perimeter of the particle.
In the invention, it is important that the particles capable of being positively and negatively charged have different colors. The shape factor of at least one of the particulate material is also important. By making such an arrangement that the two particulate materials have different colors, a density contrast can be developed across an image site having the group of particles capable of positively charged and an image site composed of the group of particles capable of negatively charged. Further, by setting the shape factor to the above defined range, a proper space occur between the particles to enhance the fluidity of the group of particles, making it possible to give a sharp distribution of triboelectricity of the particles capable being positively and negatively charged. On the other hand, the adhesion between the particles and the substrate due to the contact of the particles with the substrate having the polarity being opposite to charge of the particles decreases because a proper space exists between the positive and negative particles. In this arrangement, even prolonged repetition of rewriting, the change of image density is small and the change of density uniformity is small, making it possible to display an image having a stabilized density contrast and reduce the driving voltage required for image display.
In the image display medium of the invention, it is preferable that one of the one, which can be positively charged, and the another one, which can be negatively charged, is white. By making at least one of the particulate materials white, the coloring power and density contrast of the other particulate material can be enhanced. It is also preferable that the one, which is white, comprises a coloring material and that the coloring material is titanium oxide. In other words, the white particulate material preferably comprises a coloring material and the coloring material is preferably titanium oxide. The use of titanium oxide makes it possible to enhance the opacifying power and hence further enhance the contrast in the wavelength range of visible light.
On the other hand, the image forming device of the invention is an image forming device for forming an image on the foregoing image display medium of the invention, the image forming device has an electric field generating unit for generating an electric field between the pair of substrates according to the image to be formed.